For persons who cannot benefit from traditional, air conduction hearing aids there are other types of sound transmitting hearing aids on the market, i e bone anchored hearing aids which mechanically transmit the sound information to a persons inner ear via the skull bone by means of a vibrator. The hearing aid device is connected to an anchoring element in the form of an implanted titanium screw installed in the bone behind the external ear and the sound is transmitted via the skull bone to the cochlea (inner ear), i e the hearing aid works irrespective of a disease in the middle ear or not. The bone anchoring principle means that the skin is penetrated which makes the vibratory transmission very efficient.
This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of impaired hearing. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling or a snap in coupling. One example of this type of hearing aid device is described in U.S. Pat. No. 4,498,461 and it is also referred to the BAHA® bone anchored hearing aid marketed by Entific Medical Systems in Göteborg.
The fixtures which have been used so far for the bone anchored hearing aid devices of the type which have been mentioned here as well as for existing ear or orbital prostheses, have been designed in such a way that a screw tap is required to form an internal thread in the hole drilled in the skull bone. One example of such a fixture illustrated in U.S. Des. 294,295. This fixture has an external thread with small cutting edges with only a minor scraping effect in the pre-tapped bone hole. It has also a flange which functions as a stop against the bone surface when the fixture is screwed down into the skull bone. The flange is also in this case provided with through holes for bone ingrowth or the like.
It is also previously known to use so-called self-tapping fixtures for permanent anchorage of dental prostheses, dental implants, see for instance U.S. Pat. No. 5,064,425 and U.S. Pat. No. 5,269,685, which fixtures can be installed without the use of any screw taps. However, these types of fixtures which are used in the jaw-bone cannot be used for anchorage in the skull bone, which bone is much thinner than the jaw-bone. The dental implants (fixtures) are too long and they have very deep, longitudinal bone cavities for collecting and retaining all the cut-off bone chips material.